IQ modulation is a very general form of modulating an RF carrier to convey program information. After IQ modulation both the amplitude and phase of the modulated carrier can convey recoverable information. Although IQ modulation is sufficiently general to produce conventional analog AM and conventional analog FM modulation, it is most often used in digital settings where the program information is in digital form to start with, or if originally analog, has been digitized. For an introduction to IQ modulation in digital settings, see an article entitled An Instrument for Testing North American Digital Cellular Radios published in the April 1991 HEWLETT-PACKARD JOURNAL, pages 65-72.
FIG. 1A is a simplified block diagram of a portion of a prior art communications system incorporating an IQ modulator. As shown in that figure, program information 2 that may be in either analog form or digital form (i.e., either analog or digital electrical signals) is applied to an IQ encoder 3. Its purpose is to translate, or convert by mapping and/or encoding, the format of the program information 2 into a pair of signals I (4) and Q (5). In the case where the program information 2 is an analog signal V.sub.in, then signals I (4) and Q (5) would actually vary continuously as analog signals, also. That variation would typically be in accordance with some functions F and G, such that I=F(V.sub.in) and Q=G(V.sub.in), rather than a simple I=V.sub.in or Q=V.sub.in, although that is possible. In the case where the program information is digital, the resulting actual voltage values for I and Q are restricted to discrete possibilities. Transitions between these possibilities are in principle abrupt, and if left untreated produce unwanted and deleterious effects in the modulated signal. It is therefore usual to constrain these abrupt changes in I and Q by filtering them before they are used.
It is quite a common circumstance for the program information 2 to be in digital form. Perhaps the program information never existed in analog form at all, and was always just so many bits in the first instance; or perhaps it is a digitized representation of some analog phenomenon, say, speech. In these digital cases, it is common to group the incoming data stream into groups of, say, n bits. There are 2.sub.n different possible combinations that those n bits might have. Then what the IQ encoder 3 does is produce voltage values for I and Q that, as a combination, correspond to one of the 2.sup.n many combinations of the group. Typically, n is even, and each of the signals I 4 and Q 5 can independently assume 2.sup.(n/2) different states or voltage values, for a total of 2.sup.n combinations of I and Q taken together.
The signals I 4 and Q 5 are applied to an IQ modulator 6 that also receives an RF signal 8 produced by a local oscillator 7. The IQ modulator 6 operates upon the RF signal 8 to produce a modulated RF signal 9 whose amplitude and phase may each convey information. This signal 9 is typically amplified by an amplifier 10 to produce a transmittable signal. In the example of FIG. 1A that signal is applied to an antenna 11 to produce a radiating modulated signal 12.
For the sake of completeness, we shall touch briefly on the corresponding IQ receiver 13 shown in FIG. 1B. Typically, a radiated IQ modulated signal 14 (corresponding to radiating signal 12 in FIG. 1A) produces in an antenna 15 a corresponding electrical signal that is then amplified by an amplifier 16. The amplifier 16 produces a high level IQ signal 17 suitable for application to an IQ demodulator 18. In digital systems IQ demodulator 18 typically cooperates with a clock recovery capability and perhaps also an LO recovery circuit. In any receiver IQ demodulator 18 produces signals I 19 and Q 20 that correspond to earlier I and Q signals (4, 5) in IQ transmitter 1. These recovered I and Q signals 19 and 20 could, in a digital system, be strobed by the recovered clock signal (not shown). In more sophisticated systems, the nature of the IQ transitions produced by the filter in the IQ modulator are incorporated into a digital signal processing network that selects or predicts the most probably correct new values of I and Q, based on their recent prior history. An IQ decoder 21 translates the demodulated IQ combinations back into the original format for the program information. This re-translation appears as recovered program information 22.
As will become evident as we proceed, phase shift networks are used within the IQ modulator 6 (as well as within the IQ demodulator 18). These networks are used to split a single signal, such as the RF signal 8 from local oscillator 7, into a pair of signals that are in exact quadrature (i.e., exhibit between themselves a phase difference of ninety degrees). What is more, it is also quite desirable that the signals in quadrature also be of nearly equal amplitude, since amplitude variations can be translated into apparent phase variations by subsequent circuitry. As the data rate (bandwidth) increases the number of IQ states used also increases, meaning that there is a less pronounced difference between those states. The accuracies of the IQ modulating and IQ demodulating processes depend heavily upon accurate determination of phase. And while these requirements can be met for any particular local oscillator frequency, or for a narrow range of local oscillator frequencies, it is quite something else to meet them with an IQ modulator or IQ demodulator intended for use with a local oscillator input signal that is allowed to vary over a wide range, say, 200 MHz to over 3 GHz.
In support of this, consider the simplified block diagram of a prior art IQ modulator 23 shown in FIG. 2. An RF input signal (24, 8) serves as the signal to be modulated, and is applied to a quadrature network 25. The output of the quadrature network 25 is two signals LO.sub.i 26 and LO.sub.q 27 that are of the same frequency as the RF signal input (24, 8) but are in quadrature. These signals are applied to multipliers 28 and 29, respectively. Each of these multipliers also receives its associated I or Q input signals, denoted in the figure as I.sub.in (30, 4) and Q.sub.in (31, 5), respectively. Multipliers 28 and 29 may comprise double balanced mixers, various types of switching networks, or actual (analog or digital) multiplier circuits. The outputs 32 and 33 of the multipliers 28 and 29 are then summed in a summer 34 to produce an output signal (35, 9) that is the IQ modulated signal.
Frequency dependent amplitude variations are inherent in the reactive phase shifters used internally within the quadrature network. Limiting amplifiers are typically used to restore the phase shifted signals to a selected constant amplitude. As the signal being shifted varies in frequency the degree of limiting experienced by the limiting amplifiers varies, since the amplitude of their input signal varies accordingly. The net result is an unwanted change in the resulting phase shift in the signal as finally amplified. The amount of the unwanted phase shift will be related to the amount of amplification needed to produce limiting. But correct quadrature is essential for proper IQ modulator operation. It is because of this that conventional IQ modulators must be "tweaked" to operate at any particular frequency. But a modulator that must be tweaked is not a broadband device suitable for straightforward use over a wide range of frequencies.
This, then, is the problem: A wideband IQ modulator (or demodulator) requires accurate phase shifting of a signal to produce two signals LO.sub.i and LO.sub.q that are precisely ninety degrees apart (i.e., in quadrature). Many reactive phase shift networks disturb amplitude as they shift phase. The disturbance is a function of frequency. Signal amplitude can be subsequently restored, but at the expense of introducing some additional (and generally unknown) amount of phase shift. Since any particular mixer tends to work best over a limited range of applied power, amplitude restoration is generally necessary. This means that the LO.sub.i and LO.sub.q signals in an IQ modulator (or their LO counterparts in an IQ demodulator) will not be in exact quadrature as the frequency of operation is varied away from some optimum value. This limits the performance of the IQ modulation scheme in use, unless operation is at that optimum frequency.
Another view of the same problem is the observation that it would be desirable if a reactive phase shift network were free of amplitude variations despite variations in applied frequency. If such were available then a quadrature network and its companion amplifiers could produce exact quadrature over a broad range of applied frequencies. That would in turn allow a single part to serve in a wide variety of different applications, and allow for considerably simplified schemes for wide frequency range applications where the output of a single frequency IQ modulator must otherwise be mixed with swept or variable frequency sources.